Balancing impedance included in a fork

ABSTRACT

A simplified balancing impedance (Z 2 ) included in a transformer fork has a constant capacitor (C 5 ) connected in series with the termination resistor (R 1 ). The variable capacitance in the balancing impedance contains a resistor-capacitor network (R 2 , C 6  ; R 3  C 7 , C 8  -C 11 ) connected in parallel with the termination resistance. The constant capacitor and the center point capacitor (C 2 ) on the balancing side of the fork can thus be replaced by one capacitor.

FIELD OF INVENTION

The invention relates to a balancing impedance included in a fork, andmore particularly to a transformer fork for transforming an incomingtwo-wire line to a four-wire connection. The impedance according to theinvention preferably balances towards a resistance of a specified value(for example, 600 or 900 ohms) in series with a constant capacitance(for example, 2.16 μF) and towards a parallel capacitance, variablewithin a certain range (for example, 0-126 nF).

BACKGROUND

Transformer forks for two-wire to four wire conversion have been knownfor quite a long time. Usually two transformers are included in thefork, the transformers together forming the two-wire side, four-wireside and balancing side of the fork. In particular, it is previouslyknown to form the transformers with two center point capacitorsconnected between two windings in the transformer on its line andbalancing sides.

SUMMARY OF INVENTION

An object of a present invention is to simplify the operation of thebalancing impedance included in a transformer fork by replacing thecenter point capacitor on the balancing side of the fork and theconstant capacitance in the balancing impedance by only one capacitorthe capacitance of which is lower than the two mentioned capacitances.

According to the proposed invention this object is attained byconnecting, in parallel to the resistor which forms the terminationresistance, a network of connectable resistor-capacitor links, which areconnectable in parellel with the resistor. As a result the center pointcapacitor and the constant capacitor included in the balancingimpedance, which capacitors are connected in series, can be replaced byonly one capacitor, the capitance of which is less than the capacitanceof the center point capacitor and the constant capacitor. This achievesthat costs as well as space are saved.

The balancing impedance is then characterized by a two-wire pathincluding a capacitor connected to a two-wire line including a linecapacitance and a terminating resistance, a first and second four-wirepath and a balancing path, a balancing impedance connected to saidbalancing path, said balancing impedance including a first capacitorwhose capacitance corresponds to said two-wire line capacitance and tosaid capacitance in the two wire path and a first resistor, said firstcapacitance is connected in series with said first resistance whosevalue corresponds to the terminating resistance, a plurality of brancheseach including a capacitance are connected in parallel with a firstresistor. A switch arrangement including each of the branches forselectively connecting each of the branches to the first capacitance atleast one of the branches including a further resistor connected inparallel with an associated capacitor for simulating the linecapacitance.

BRIEF DESCRIPTION OF DRAWING

The invention will be further described with reference to the attacheddrawing wherein

FIG. 1 shows a prior art transformer fork of known construction; and

FIG. 2 shows the circuit diagram for the balancing impedance accordingto the invention connected to the balancing side of the fork accordingto FIG. 1.

DETAILED DESCRIPTION

In FIG. 1, a fork circuit of a conventional construction is shown. Theline side of the fork includes four windings 1₁ -1₄ and a center pointcapacitor C₁ is connected between the two windings 1₂ and 1₃. The twofour-wire sides are constituted by the windings 1₅ and 1₆. The windings1₇ and 1₈ are connected via the capacitor C₂ and form the balancingside. To the end points of these windings a balancing impedance Z₁ isconnected consisting of the constant capacitor C₃ in series with theresistor R and, parallel to these, the variable capacitor C₄. To obtaingood balance, it is valid that C₁ =C₂. The capacitors C₁ and C₂ usuallyare of the magnitude 1-2 μF. The capacitors C₂ and C₃ are substantiallyconnected in series, since the capacitance of the variable capacitor C₄is low (usually 0-0.126 μF). It would be desirable to join thecapacitors C₂ and C₃ to one capacitor, since the series connection of,for example, 1.15 μF and 2.16 μF is 0.75 μF, which is much less than thesum 1.15+2.16=3.31 μF. The variable parallel capacitance C₄, however,prevents this. The capacitor C₄ is usually constructed as a network ofcapacitors connected in parallel, each in series with a switch. If thisnetwork could be connected in parallel with the resistor R, saidsimplification could be effected. The resistor R should be equal to theterminating resistance of the line, usually 600 or 900 ohms.

According to the proposed invention this problem will be solved byconnecting a network of parallel resistor-capacitor links in parallelwith the resistor which constitutes the terminating resistor. FIG. 2shows an embodiment of the invention. The line side of the forktransformer is excluded for the sake of simplicity. The capacitor C₃ inFIG. 1 is replaced by a capacitor C₅ connected in series with the centerpoint capacitor C₂. Thus the capacitors C₂ and C₅ can be replaced by onecapacitor. The series capacitor C₅ is included in the balancingimpedance Z₂ and is connected to the end point of the winding 1₇ and toa resistor R₁ connected to the end point of the second winding 1₈included in the fork transformer. In order to replace the constantcapacitor C₃ according to FIG. 1 with the capacitor C₅, a capacitornetwork is connected, in parallel with the resistor R₁ with a number ofconnectible branches corresponding to the variable capacitor C₄ inFIG. 1. In the embodiment according to FIG. 2, each of two branchesconsists of a resistor R₂ and R₃ respectively, in parallel with acapacitor C₆ and C₇ respectively, the parallel connection beingconnected in series with the switches k₁, k₂. In addition there arefurther parallel branches, each including a capacitor C₈ -C₁₁ in serieswith the switches k₃,k₄,k₅ and k₆ respectively.

The balancing impedance Z₁ according to FIG. 1 should have the sameimpedance value as the balancing impedance Z₂ according to FIG. 2, seenfrom the two end points of the windings I₁ and I₂. This gives thefollowing conditions: ##EQU1## This equation is satisfied when: ##EQU2##and R_(y) =the resistance of the connected resistors R₂, R₃ according toFIG. 2. ##EQU3## The capacitance values C₄ represent the variablecapacitors according to FIG. 1. These values should be simulated asclose as possible by the capacitance C. According to (3) thiscapacitance is dependent on both the capacitances C₃ and C₄. Since##EQU4## it is obtained from (1) that ##EQU5## which is fulfilled for##EQU6## If thus the resistor R₁ in FIG. 2 were to have the same valueas the resistor R in FIG. 1, resistors R₂, R₃ having the resistanceR_(y) determined according to (4) would have to be connected in parallelwith the resistor R₁. If, furthermore, the value of the capacitance C ischanged by connecting the capacitors C₆ and C₇, resistors R₂ and R₃respectively must at the same time be connected in parallel with theresistors R₁ according to (1) and (3). The equation (2) indicates howmuch the capacitance C₅ of the constant capacitor differs from theoriginal value C₃ according to FIG. 1. The following table can be set upfor certain values of the capacitances C₅ and C and the resistor valuesof the resistor R₁ based on values of the capacitances C₃ and C₄,usually used (according to FIG. 1):

    ______________________________________                                        C.sub.4  C.sub.5                                                                              C            R.sub.x /R                                                                          R.sub.y /R                                 ______________________________________                                        0.002    2.162  0.0020       0.998 540                                        0.004    2.164  0.0040       0.996 270                                        0.008    2.168  0.0080       0.993 135                                        0.016    2.176  0.0161       0.985 67.3                                       0.032    2.192  0.0325       0.971 33.5                                       0.064    2.224  0.0659       0.943 16.6                                       0.126    2.286  0.133        0.893 8.33                                       ______________________________________                                    

The values of the capacitance C₅ according to the table are obtainedaccording to the equation (2) with C₃ =2.16 μF. Choosing a value C₅=2.20 μF of the capacitor C₅ in FIG. 2 a sufficiently good approximationof the constant capacitance is obtained and the series connection of thecapacitor C₂ according to FIG. 2 gives a value ≈0.75 μF. R_(x) in thetable above represents the resistance of the parallel connection of theresistor R₁ and the resistor or resistors R₂, R₃ which are connectedtogether with the associated capacitors C₆ and/or C₇. The capacitancevalues of the capacitors C₆ -C₁₁ can be chosen from the so called E12-series i.e., C₁₁ =0.0022, C₁₀ =0.0039, C₉ =0.0082, C₈ =0.015, C₇=0.033, C₆ =0.068 μF, the sum of which will be 0.130 μF, which gives agood adaptation to desired values of C. A correct value of the resistorR₁ according to the equation (1) above is obtained by connection of theresistors R₂, R₃. Regarding the lower capacitances, the proportion R₁/R₂, R₁ /R₃ is very small for which reason no resistors connected inparallel with the capacitors C₈ -C₁₁ (=0.0002-0.004 μF) are necessary.R_(y) in the table represents the resistance to be connected in parallelwith the capacitors C₆, C₇, i e R₂ and R₃ respectively. The proportionR_(y) /R thus indicates how much greater R_(y) should be relative to Rfor the chosen capacitances C₆, C₇. With the aid of the table suitableresistance value of R₂,R₃ can be estimated.

By the proposed balancing impedance, a capacitor with low values hasbeen replaced by two capacitors with higher capacitance value at thecost of a few number of resistors. Since resistors are cheaper thancapacitors, a total saving of costs is obtained.

I claim:
 1. A balance impedance in a transformer hybrid circuit, said circuit including a two-wire path including a capacitor and, connected to said capacitor, a two-wire line including a line capacitance and a terminating resistance, a first and a second four-wire path and a balancing path, a balancing impedance connected to said balancing path, said balancing impedance including a first capacitor (C5) whose capacitance corresponds to said two-wire line capacitance and to said capacitance in the two-wire path, a first resistor, said first capacitor being connected in series with said first resistor (R1) whose value corresponds to said terminating resistance, a plurality of branches each including a capacitor (C6, C7, . . . ) connected in parallel with said first resistor and switch means (k1, k2, . . . ) included in said branches for selectively connecting each of said branches to said first capacitor, at least one of said branches including further resistor (R2, R3) connected in parallel with the capacitor in said one branch for simulating the line capacitance.
 2. A balancing impedance according to claim 1 wherein the capacitance values of the respective capacitors in the respective branches have gradually decreasing values.
 3. A balancing impedance according to claim 1 wherein the capacitance values of the respective capacitors in the respective branches have gradually increasing values.
 4. A balancing impedance according to claim 1 wherein the resistance of the resistors in the respective branches have gradually increasing values.
 5. A balancing impedance according to claim 1 wherein the resistance of the resistors in the respective branches have gradually decreasing values.
 6. A balancing impedance according to claim 1 wherein the proportion between the resistance values of the resistors (R₂, R₃) in two of said branches equal the inverted value of the proportion between the capacitance values of of respective capacitor (C₆, C₇) in the associated branches. 